In recent years, the focus of pavement engineering has shifted from design and construction of new highways to preventive maintenance and rehabilitation of the existing highways. A highway maintenance program is usually based on a visual condition survey and, to a lesser extent, on appropriate in situ tests. By the time symptoms of deterioration are visible, major rehabilitation or reconstruction is often required. If the onset of deterioration can be measured accurately in the early stages, the problem can often be resolved or stabilized through preventive maintenance.
The Strategic Highway Research Program has identified six broad elements that cause and contribute to pavement deterioration, as discussed in Maser, K. R. and M. J. Markow, "Measuring Systems and Instrumentation for Evaluating the Effectiveness of Preventive Maintenance," Report SHRP-MIUWP-91-513, Strategic Highway Research Program, National Research Council, Washington, D.C. (1990), the disclosure of which is herein incorporated by reference:
1. Pavement moisture; PA0 2. Subsurface problems or discontinuities; PA0 3. Voids or loss of support under rigid pavements; PA0 4. Overlay delamination; PA0 5. Fine cracking; and PA0 6. Asphalt aging. PA0 1. Moisture in the base layer (flexible pavement); PA0 2. Voids or loss of support under joints (rigid pavement); PA0 3. Overlay delamination; PA0 4. Fine cracking; and PA0 5. Pavement aging. PA0 1. Ultrasonic Body Wave; PA0 2. Ultrasonic Surface Wave; PA0 3. Impulse Response; PA0 4. Spectral Analysis of Surface Waves (SASW); and PA0 5. Impact Echo.
A. Pavement Conditions
1. Moisture in the Foundation
The types of distress caused by moisture-related problems in the foundation layers (in advanced stages) are summarized by Carpenter et al., "A Pavement Moisture Accelerated Distress Identification System--Volume 2: User's Manual," Research Report FHWA-RD-81-080 Federal HighWay Administration, U.S. DOT, Washington, D.C. (1981), the disclosure of which is herein incorporated by reference. Typically, the softening of one or more of the foundation layers and the degradation of material quality in terms of stiffness and strength are the initial manifestations of excess moisture within the pavement system. Field studies have shown that wheel loads on saturated sections are many times more damaging than those on dry sections. See Cedergren, H. R., Drainage of Highway and Airfield Pavements (1974), the disclosure of which is herein incorporated by reference.
2. Moisture Under Joints
The deterioration of foundation layers exposed to moisture in a rigid pavement is similar to that in a flexible pavement. Erodible foundation materials under a slab will deteriorate when subjected to load. The existence of moisture significantly increases the rate of deterioration. In this case, the foundation layer will either become softer or a void will develop under the concrete slab. Slab curling (bending or warping) will contribute to the deterioration.
3. Voids or Loss of Support
The presence of voids or loss of support underneath a slab of rigid pavement is detrimental because it causes increase in stresses in pavement, decrease in the fatigue life of the pavement, and a possibility for faulting of joints. Important factors in this process are discussed by Torres, F., and B. F. McCullough, "Void Detection and Grouting Process," Research Report 249-3, Center for Transportation Research, The University of Texas, Austin, Tex. (1983), the disclosure of which is herein incorporated by reference. The larger the void or the thinner the slab, the lower the support and the shorter the life of the pavement.
4. Overlay Delamination
The process and significance of overlay delamination is well known. The degree of interfacial bonding influences the state of stress within the overlay. Interfacial bonding has been identified as the most significant factor affecting overlay performance. See Ameri-Gaznon, M., and D. N. Little, "Permanent Deformation Potential in Asphalt Concrete Overlay Over Portland Cement Concrete Pavements," Research Report 452-3F, Texas Transportation Institute, College Station, Tex. (1988), the disclosure of which is herein incorporated by reference. When delaminated, the overlay acts independently of the rest of the pavement system, allowing excessive movement at the bottom of the overlay relative to the top, where the wheel load is in contact with the pavement. As a result, large tensile strains develop at the bottom of the overlay.
5. Fine Cracking
Cracks often begin as hairline cracks that allow little water into the structure. Although they are a discontinuity in the pavement structure, they are not generally a problem until they become wide enough to allow water to enter into the structure. If allowed to deteriorate, they become wider, allowing much more water to enter. Furthermore, the intrusion of an incompressible material during a cold period creates high compressive forces on the crack or joint face during warmer periods, creating spalling. The cracks may also contribute to the aging process of asphalt on the crack face, accelerating crack deterioration.
6. Pavement Aging
The aging process in the field is complex. Several independent investigations have indicated that the stiffness of the asphalt in a flexible paving layer increases with time (aging). See Tia, M. et al., "Investigation of Original and In-Service Properties for Development of Improved Specifications: Final Phase of Testing and Analysis," Final Report. Engineering and Industrial Experiment Station, University of Florida, Gainsville, Fla. (1988); Goodrich, J. L., "Asphalt and Polymer Modified Asphalt Properties Related to Performance of Asphalt Concrete Mixtures," Proceedings, Association of Asphalt Paving Technologists vol. 57, pp. 116-75 (1988); Von Quintus, H. et al., "Asphalt Aggregate Mixture Analysis System," Report 338 (National Cooperative Highway Research Program, National Research Council, Washington, D.C.) (1991), the disclosures of which are herein incorporated by reference. It has further been suggested that aging should be considered in two stages: short-term and long-term. See Bell, C. A, "Summary Report on the Aging of Asphalt-Aggregate Systems," Report SHRP-A-305. Strategic Highway Research Program, National Research Council, Washington, D.C. (1989), incorporated herein by reference. Short-term aging occurs during construction, while the mix is hot. Such aging is mainly due to a loss of volatile components in the asphalt. Long-term aging occurs after the mixture is in place, and is primarily attributed to progressive oxidation of the material in the field.
B. Maintenance Activities
A device capable of detecting the foregoing elements early is desirable for preventive maintenance. Four major features in such a measurement device are necessary for effective maintenance measurements. First, the device should be sensitive enough to measure a contributing factor to a potential distress "soon enough." Second, the measurements should be accurate and comprehensive enough to identify the layer contributing to a potential distress. Third, the device should be precise enough to verify the effectiveness of maintenance processes. Finally, the device should be sophisticated enough to differentiate between a rehabilitation activity and a maintenance activity. Rehabilitation includes complete or partial replacement of the pavement layer, whereas maintenance involves fixing (e.g., patching) a more localized defect prior to total tiff lure. By way of analogy, pavement rehabilitation is to pavement maintenance as fitting dentures is to filling a cavity. Generally, maintenance is less expensive than rehabilitation.
The maintenance activities discussed herein are those that correct a localized area of deterioration, preserve the existing pavement, and reduce the rate of deterioration (e.g., corrective and preventive maintenance). Treatments that fall within this category of maintenance are included in Table I for two common types of road surfaces: Asphaltic concrete and Portland cement concrete. In general, these treatments do not increase the structural or traffic-handling capacity of the roadway.
TABLE I ______________________________________ Treatments that Are Considered Maintenance Asphaltic-Concrete Roads Portland-Cement Concrete Roads ______________________________________ Patching Patching Crack Sealing or Filling Joint Repair Surface Sealing Crack and Joint Sealing (all types) Undersealing ______________________________________
Each of these activities addresses specific problems in the pavement structure. To determine when to apply a maintenance treatment and which treatment is appropriate, the maintenance engineer tries to answer several questions. Does this section of pavement need a treatment now? If not, will it need one in the near future (less than three years)? Is the problem localized, or does it cover a large area? Which treatment should be applied? Is the treatment cost effective?
1. Patching
Patching is the repair of localized areas of low strength or other types of deterioration. Patching can address every possible type of deterioration, but only if the damage is localized. Such distress can occur in any pavement layer, leading to a localized failure observable on the surface. Loss of strength can be caused by a change in the material properties or localized differences in construction and original materials. It can also be due to loss of support caused by erosion or degradation of supporting layers.
Maintenance engineers typically do not test to determine if patching is required. They simply begin applying patches when the deterioration affects the pavement surface to the point that driving becomes hazardous. The key question that a maintenance engineer needs to address in testing is whether patching will effectively resolve the problem. If the problem is widespread, a comprehensive rehabilitation treatment will be more cost effective than patching.
2. Crack and Joint Sealing
Many of the materials used in pavement construction have moisture-sensitive stiffness. As the moisture content of unbound granular materials and soils increases, their stiffness decreases. Moisture leads to the degradation of asphalt concrete due to stripping, aging, weathering, and raveling. Free water under Portland cement slabs can develop very high pressures, eroding the base and subbase materials, or leading to loss of support. Crack and joint sealing help to prevent such deterioration by reducing the influx of moisture from the surface into the pavement structure.
A typical maintenance engineer will generally use the observable condition of the crack and joints to determine if crack and joint sealing is appropriate. Many maintenance engineers will not seal a crack until it is greater than 5 mm wide. If the amount of weakening resulting from moisture at the joints and cracks could be determined, that information could help determine when crack and joint sealing is needed to reduce the infiltration of moisture.
3. Surface Seals
Surface seals generally extend the life of pavements by improving the surface friction of the pavement, by reducing weathering and raveling, or by reducing the infiltration of moisture into the pavement structure.
The maintenance engineer normally looks for signs of weathering and raveling or for the presence of a network of fine cracks that can be sealed with the surface seal. If the presence and level of aging could be determined, the degradation of asphalt because of aging could be prevented or reduced. If the degradation of paving materials because of abnormal moisture levels in the asphalt and supporting layers or fine cracking could be determined, the need to place a seal to reduce infiltration of water into the structure could be evaluated.
4. Undersealing
Undersealing is the process of filling voids under Portland cement concrete pavements with a grout of cementatious material in order to reestablish support under the slab. The movement and loss of fine-grained materials creates voids, normally on the leave side of the joint or crack. This leads to faulting of the joint or crack. The loss of support also increases the stress in the Portland cement concrete pavement near the corners, leading to corner breaks.
In some cases, the base material is degraded but not ejected. This can create a thin layer of very soft material under the joint. A loss of support is then present without a true void. In such a case, undersealing generally cannot displace the deteriorated materials sufficiently to reestablish full support.
A maintenance engineer typically looks for the presence of pumping, faulting, and corner breaks to determine that voids are present and to determine if undersealing is appropriate. In some instances, nondestructive testing devices (such as falling weight deflectometer and Dynaflect) or manual methods (such as dropping a BB) are used for this task. Measurements to determine the loss of support, the presence of voids, and the size of voids are needed to determine if voids are developing and if undersealing should be considered to reestablish support.
C. Developing Specifications for Measurement
It is extremely important to measure the precursors of distress in the early stages. Below a certain measurement level, a change in the distress-triggering parameters results in insignificant changes in the condition or serviceability of the pavement. However, if the precursors of distress are identified "too late," reconstruction or rehabilitation may be more appropriate.
The remaining life of a pavement is controlled by the complex interaction of several factors such as traffic, pavement structure, drainage, road geometry, climate, and economy. In recent years, several methods have been developed to predict the type and rate of deterioration and to suggest alternative maintenance strategies at appropriate time intervals.
An example of a maintenance method is the Texas Flexible Pavement System (TFPS), described in Uzan, J., and R. E. Smith, "TFPS Technical Report," Draft Report for Research Project 455, Texas Transportation Institute, Texas A&M University, College Station, Tex. (1988), and Rodhe, G. et al., "User's Guide to the Texas Flexible Pavement System (TFPS) Program," Texas Transportation Institute, College Station, Tex. (1990), the disclosures of which are herein incorporated by reference. In ideal conditions, one can adhere to these "theoretical" maintenance schedules. Often, however, pavements experience distress prematurely or maintenance activities are ineffective.
Cracking and moisture in the foundation are considered together because of the strong interaction that exists between them. The paving layer of a new pavement (which comprises a top layer, a base, and, optionally, a subbase, on top of a subgrade) is usually impervious, and cracks are scarce or nonexistent. In this stage, most of the damage to the pavement is the result of traffic or environment, and moisture infiltrates either from the shoulders or from the water table. As soon as cracks develop, moisture may penetrate from the surface. If the surface layer is primarily a wearing course and the majority of the structure is in the base, the infiltration of moisture and the existence of cracks are not of great concern so long as the base and subgrade materials do not lose their integrity because of exposure to moisture.
The TFPS considers the effects and interactions of several parameters in a comprehensive fashion. Factors that are considered (excluding political and economic factors) include the climatic parameters (such as the amount and seasonal distribution of rainfall), the drainage properties of the base and subgrade materials, the structural properties of the asphalt concrete, base, subgrade, and their seasonal variations, and the nature and seasonal distribution of traffic. The amount and seasonal distribution of the rainfall are modeled from the historical data from each county in the State of Texas. Based upon these climatic models, the properties of each layer are regularly modified and updated.
The parameters that the TFPS considers include moisture, temperature, and distress type; however, the TFPS program does not consider the transient and dynamic nature of change in moisture or stiffness with time. Considering the most basic principles of geotechnical engineering, the transient and dynamic nature of change in moisture is of little practical use in predicting maintenance life. A material that becomes saturated and unsaturated over a short time period is a well-drained material and has high permeability. The strength and stiffness of such a material (and as a result, the remaining life of a pavement constructed with or over such a material) is not significantly affected by change in moisture. On the other hand, in a material that does not exhibit large fluctuation in moisture over short periods (i.e., a material with low permeability), the change in equilibrium base moisture may significantly affect its stiffness and remaining life.
Thus, a shortcoming of TFPS, as applied to maintenance problems, is that it does not model the accumulation of damage resulting from change in equilibrium base moisture or stiffness.
In summary, maintenance engineers have typically relied on visible distress, along with pavement age and traffic levels, to schedule preventive or corrective maintenance. Preventive maintenance usually costs only a fraction of the expense of major corrective maintenance. However, successful preventive maintenance requires diagnosing pavement distress at its earliest stages. Objective engineering information on this sort of "distress precursor" has not been readily available, in part due to the difficulty of measuring pavement damage at early stages. Thus, a need exists for a reliable system to nondestructively test pavement for the presence of visible as well as invisible distress precursors and to determine when and what sort of preventive maintenance is needed.